U.S. President Barack Obama and with Estonian President Toomas Hendrik Ilves review the honor guard at Kadriorg Palace in Tallinn, Estonia, Wednesday, Sept. 3, 2014. Obama is in Estonia for a one day visit where he will meet with Baltic State leaders before heading to the NATO Summit in Wales. (AP Photo/Charles Dharapak)

U.S. President Barack Obama and with Estonian President Toomas Hendrik Ilves review the honor guard at Kadriorg Palace in Tallinn, Estonia, Wednesday, Sept. 3, 2014. Obama is in Estonia for a one day visit where he will meet with Baltic State leaders before heading to the NATO Summit in Wales. (AP Photo/Charles Dharapak)

U.S. President Barack Obama greets children as he is welcomed by Estonian President Toomas Hendrik Ilves at Kadriorg Palace in Tallinn, Estonia, Wednesday, Sept. 3, 2014. Obama is in Estonia for a one day visit where he will meet with Baltic State leaders before heading to the NATO Summit in Wales. (AP Photo/Charles Dharapak)

U.S. President Barack Obama steps off Air Force One as he arrives in Tallinn, Estonia, Wednesday, Sept. 3, 2014, for a one day visit where he will meet with Baltic State leaders before heading to the NATO Summit in Wales. (AP Photo/Charles Dharapak)

U.S. President Barack Obama is greeted by Estonia’s Minister of Foreign Affairs Urmas Paet, left, and Estonia’s Chief of Protocol Toomas Kahur, center, as he arrives in Tallinn, Estonia, Wednesday, Sept. 3, 2014, for a one day visit where he will meet with Baltic State leaders before heading to the NATO Summit in Wales. (AP Photo/Charles Dharapak)

U.S. President Barack Obama reviews the honor guard as he is welcomed by Estonian President Toomas Hendrik Ilves at Kadriorg Palace in Tallinn, Estonia, Wednesday, Sept. 3, 2014. Obama is in Estonia for a one day visit where he will meet with Baltic State leaders before heading to the NATO Summit in Wales. (AP Photo/Charles Dharapak)

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TALLINN, Estonia (AP) â€” Mounting a show of solidarity with NATO allies, President Barack Obama announced plans Wednesday to send more Air Force units and aircraft to the Baltics, as he sought to reassure nations on edge over Russia’s aggression in Ukraine.

With Moscow supporting pro-Russian separatists in eastern Ukraine, NATO allies such as Estonia fear they could be the next target, and Obama’s one-day visit to Estonia was designed to emphasize the U.S. commitment to defending its allies and ramp up consequences for Russian President Vladimir Putin. Obama, ticking through a list of U.S. military resources already at work in the region, said the U.S. has a duty under the NATO charter to the alliance’s collective defense.

“It is unbreakable, it is unwavering, it is eternal. And Estonia will never stand alone,” Obama said at a joint news conference with Estonian President Toomas Hendrik Ilves.

Standing shoulder to shoulder with Estonia’s president, Obama called Estonia’s Amari Air Base an ideal location to base those additional forces, which come as NATO nations prepare to bolster a rapid-response force for the region.

Obama held up Estonia as an example of how every member of the military alliance needs to do its fair share for the collective defense of all 28 members. The U.S. and Estonia are two of four NATO countries that fulfill their pledges to contribute 2 percent of their GDP to defense spending.

Obama’s visit to Estonia precedes a NATO summit this week at which allies will commit to a more robust response to Russia’s incursion in Ukraine.